1. Field of the Invention
This invention relates to monopole antennas and in particular to wideband monopole antennas which are reduced in size.
2. Description of the Prior Art
Monopole antennas are generally constructed as straight, vertical wires mounted above the ground plane and fed with a coaxial line at the input. As shown in FIG. 1, the narrowband monopole antenna 2 is fed by a coaxial line 4 and resonates when the length of the antenna above the ground plane 6 is equal to a quarter of a wavelengty .lambda./4.
Since .lambda. is inversely related to frequency, as frequency decreases, .lambda. increases. Thus, a wave wight a frequency of 300 MHz and a .lambda. of about 1 meter requires a resonant monopole antenna about 0.25 meters long. A significant shortcoming of conventional monopole antennas is that it cannot be used in situations where space is limited.
For example, a resonant antenna located within the body of a spacecraft may be required to fit within a space which is about 1 meter square. Since a 50 MHz wave has a .lambda. of 6 meters, a conventional resonant monopole antenna must be .lambda./4 in length or about 1.5 meters. That dimension would clearly exceed the dimensions of the hypothetical spacecraft.
A second shortcoming of a conventional monopole antenna is that it can function only as a narrow band antenna. While wide band performance can be achieved by adding additional single monopole antennas, these additional antennas require more space, further limiting the use of conventional monopole antennas.
A great deal of research and development has been devoted to the development of size-reduced, wideband antennas. However, none of the size-reduced, wire-type antennas possess wide bandwidth impedance characteristics. Previous works on reduced height monopoles include: (1) wire structure with top loading as a T structure or inverted L configuration (E. C. Jordan, Electromagnetic Waves and Radiating Systems, Ch. 14, "Antenna Practice and Design," p. 512, Prentice-Hall, 1950); (2) helical monopoles constructed with a wire wound in a helic (M. Eovine, "Helical Monopole HF Antenna", Chu Associates, Littleton, MA, Final Report, 15 Oct. 1962, DDC No. AD 288270); (3) .lambda./8 monopole blade antennas with strip transmission line radiators and resonant broadbanding circuits (T. Kitsuregawa, Y. Takeichi, M. Mizusawa, "A One Eighth Wave Broadband Folded Unipole Antenna," Electromagnetic Theory and Antennas, Symposium Proceedings, Copenhagen, Denmark, June 1962, Vol. 6, Pt. 2, Pergamon Press. Ed. E. C. Jordan); (4) a variable length, multi-element monopole with wave traps (W. A. Edson, "Broadband Trapped Multiple-Wire Antennas," IEEE Symposium Digest, Antennas and Propagation Society, 1981, pp. 586-589) and; (5) a .lambda./8 wide-angle conical monopole (C. H. Papas, R. W. P. King, "Input Impedance of Wide Angle Conical Antennas Fed by a Coaxial Line," Proc. IRE, November 1949, pp. 1269-1271). Although all of the past wire-class monopole results approach a 50% reduction in height, they have a limitation of a narrow bandwidth response.
It is therefore an object of this invention to provide an antenna system which is reduced in size from a single monopole antenna.
It is also an object of this invention to provide a reduced size antenna system which has a wide bandwidth response.
It is another object of this invention to provide an antenna system which is efficient and has a wide useable voltage standing wave ratio (VSWR) and pattern bandwidths.
It is a further object of this invention to provide an antenna system design which is easily fabricated and can be readily varied to optimize antenna efficiency and desired performance characteristics.